MICROCONTROLLER INTERFACING CIRCUITSoku.ozturkibrahim.com/docs_stm32f4/Microcontroller...MICROCONTROLLER INTERFACING CIRCUITS revolution Revolution Education Ltd. Business Innovation

11111 Section 3Section 3Section 3Section 3Section 3

MICROCONTROLLER INTERFACING CIRCUITS

revolution Revolution Education Ltd. Business Innovation Centre, Innova Park, Mollison Avenue, Enfield, Middlesex, EN3 7XUTel: 020 8350 1315 Fax: 020 8350 1351 Email: [email protected] Web: www.rev-ed.co.uk

© copyright 2000

MICROCONTROLLERINTERFACING CIRCUITS

What is a PIC Microcontroller?What is a PIC Microcontroller?What is a PIC Microcontroller?What is a PIC Microcontroller?What is a PIC Microcontroller?

A PIC microcontroller is a single integrated circuit small enough to fit in the palm of a

hand. ‘Traditional’ microprocessor circuits contain four or five separate integrated

circuits - the microprocessor (CPU) itself, an EPROM program memory chip, some

RAM memory and an input/output interface. With PIC microcontrollers all these

functions are included within one single package, making them cost effective and easy

to use.

PIC microcontrollers can be used as the ‘brain’ to control a large variety of products. In

order to control devices, it is necessary to interface (or ‘connect’) them to the PIC

microcontroller. This section will help to enable those with limited electronics

experience to successfully complete these interfacing tasks.

Interfacing to the PIC MicrocontrollerInterfacing to the PIC MicrocontrollerInterfacing to the PIC MicrocontrollerInterfacing to the PIC MicrocontrollerInterfacing to the PIC Microcontroller

This section explains how to interface many different input and output devices to the

PIC microcontroller. BASIC code examples are provided for users of the Basic Stamp or

PICAXE systems. Explanations of BASIC commands are provided in the Commands

section (available separately). The interfacing circuits can also be used with any PIC

microcontrollers such as the PIC16F84, although these microcontrollers may require

programming in assembler code.

This section is split into four subsections:

• Introduction to ‘standard’ interfacing circuits

• Output Device Interfacing

• Input Device Interfacing

• Advanced Component Interfacing




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Note on the BASIC Code SamplesNote on the BASIC Code SamplesNote on the BASIC Code SamplesNote on the BASIC Code SamplesNote on the BASIC Code Samples

Simple BASIC code examples are provided within each subsection. The samples are

not ‘complete’ programs but sections of code that can be included within a main

program when using that particular component. When using these code samples it

must be remembered that:

1. Each pin should be set up as an input or output before using the code (stamp

users only).

2. If the hardware pins are changed from those given in the circuit diagrams it will

be necessary to modify the pin numbers in the code.

3. Any let dirs = or let pins = commands will adjust all 8 pins, in the port.

4. Try to keep variables independant of each other. If a sub-procedure uses a

variable, do not use the same variable anywhere else in the code. If the same

variable must be used again, make sure there is no way it can clash with any

other part of the code. This is the most common way of adding ‘hard-to-find’

bugs into software code.

Note on Component SelectionNote on Component SelectionNote on Component SelectionNote on Component SelectionNote on Component Selection

For convenience and ease of understanding, a single device has been adopted when

using standard interfacing components such as transistors and MOSFETS. For instance,

the ‘standard’ transistor selected is the darlington device BCX38B. This does not mean

that this device is the only transistor that can be used in all the transistor circuits, as it

is not, but it is chosen because it is suitable for the majority of project work

applications. All components listed are common devices that can be purchased from

almost all electronics distributors.




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Standard Interfacing CircuitsStandard Interfacing CircuitsStandard Interfacing CircuitsStandard Interfacing CircuitsStandard Interfacing Circuits

1. The Standard Transistor Interfacing Circuit

2. Using a Darlington Driver IC

3. The Standard Relay Interfacing Circuit

4. The Standard FET Interfacing Circuit

Output Device InterfacingOutput Device InterfacingOutput Device InterfacingOutput Device InterfacingOutput Device Interfacing

1. LED

2. Signal Lamp

3. Buzzer

4. Piezo Sounder & Speakers

5. Solar & DC (“toy”) Motors

6. Unipolar Stepper Motor

7. Bipolar Stepper Motor

8. Radio-Control Servo

9. Counter Module

10. Seven Segment Display

11. Solenoid & Isonic Solenoid Valve

12. Smart Wire / Springs

Input Device InterfacingInput Device InterfacingInput Device InterfacingInput Device InterfacingInput Device Interfacing

1. Switches

2. Potentiometers

3. Light Dependant Resistor (LDR)

4. Thermistor

Advanced Component InterfacingAdvanced Component InterfacingAdvanced Component InterfacingAdvanced Component InterfacingAdvanced Component Interfacing

1. Liquid Crystal Display (LCD)

2. Serial Communication with a Computer




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STANDARD INTERFACING CIRCUITS

Standard Circuits 1 - The Transistor Interfacing CircuitStandard Circuits 1 - The Transistor Interfacing CircuitStandard Circuits 1 - The Transistor Interfacing CircuitStandard Circuits 1 - The Transistor Interfacing CircuitStandard Circuits 1 - The Transistor Interfacing Circuit

Many output devices will require a transistor switching circuit. In most cases a

darlington pair formed from two transistors is ideal.

However this circuit requires that two separate transistors are used. It is possible to

buy a device that contains the two transistors in a single package. This transistor is

called the BCX38B, and can switch currents up to 800mA. This is the transistor used in

all the circuits through this book.

Note that it is usual to connect a back emf suppression diode across the output device.

This is essential with devices such as relays, solenoids and motors which create a back

emf when power is switched off. The diode type 1N4001 is the device recommended.

0V

Pin10k

Outputdevice

BC639

Back EMFsuppressiondiode

V+

BC548B

0V

Pin10k

Outputdevice

BCX38B

V+




© copyright 2000

0V

V+

0V

M

M

Pin 1

Pin 2 ULN

2003

In 1

In 2

In 3

In 4

In 5

In 6

In 7

Gnd

Out 1

Out 2

Out 3

Out 4

Out 5

Out 6

Out 7

V+

1

8

16

9

0V

V+

0V

M

M

Pin 1

Pin 2 ULN

2803

In 1

In 2

In 3

In 4

In 5

In 6

In 7

In 8

Gnd

Out 1

Out 2

Out 3

Out 4

Out 5

Out 6

Out 7

Out 8

V+

1

9

18

10

Standard Circuits 2 - Using a Darlington Driver ICStandard Circuits 2 - Using a Darlington Driver ICStandard Circuits 2 - Using a Darlington Driver ICStandard Circuits 2 - Using a Darlington Driver ICStandard Circuits 2 - Using a Darlington Driver IC

If a number of output devices are being controlled it may be necessary to use a

number of output transistors. In this case it will often be more convenient to use a

ULN2003 Darlington driver IC. This is simply a 16 pin ‘chip’ that contains 7

darlington transistors similar in value to the BCX38B. The ‘chip’ also contains internal

back emf suppression diodes and so no external 1N4001 diodes are required.

A device called the ULN2803 Darlington Driver IC is also available. This is identical to

the ULN2003 except that it is an 18 pin device and contains 8 darlington pairs instead

of 7. If it is necessary to pass relatively high currents through a device it can be useful

to ‘pair up’ drivers as shown with this circuit.

A ULN2803 darlington driver is supplied prefitted to the PICAXE interface board.




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Standard Circuits 3 - The Relay Interfacing CircuitStandard Circuits 3 - The Relay Interfacing CircuitStandard Circuits 3 - The Relay Interfacing CircuitStandard Circuits 3 - The Relay Interfacing CircuitStandard Circuits 3 - The Relay Interfacing Circuit

A relay can be used to switch higher power devices such as motors and solenoids. If

desired, the relay can be powered by a separate power supply, so, for instance, 12V

solenoids can be controlled by the microcontroller. Note the use of a back emf

suppression diode across the relay contacts. This is to prevent damage to the transistor

when the relay switches off. Diode type 1N4001 is suitable for this diode.

0V

Pin10k

BCX38B

1N4001 RL1

5V

Standard Circuits 4 - The Power MOSFET Interfacing CircuitStandard Circuits 4 - The Power MOSFET Interfacing CircuitStandard Circuits 4 - The Power MOSFET Interfacing CircuitStandard Circuits 4 - The Power MOSFET Interfacing CircuitStandard Circuits 4 - The Power MOSFET Interfacing CircuitPower MOSFETs can be used instead of darlington transistor pairs to switch medium

power devices. The standard MOSFET circuit is shown below. The device IRF530 is a

suitable power MOSFET to use in this circuit.

Note that it is usual to connect a back emf suppression diode across the output device.

This is essential with devices such as relays, solenoids and motors which create a back

emf when power is switched off. The diode type 1N4001 is the device recommended.

0V

Pin

+6V

IRF530

M1N4001




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OUTPUT DEVICE INTERFACING

Output Device 1 - Light Emitting Diode (LEDs)Output Device 1 - Light Emitting Diode (LEDs)Output Device 1 - Light Emitting Diode (LEDs)Output Device 1 - Light Emitting Diode (LEDs)Output Device 1 - Light Emitting Diode (LEDs)

The PIC Microcontroller can sink (“absorb”) or source

(“give out”) a small amount of current, which means that

an LED can be connected directly to the output pin. A

series resistor (value 330R) is also required to limit the

current.

LED connected to Ground Rail.

To switch on LED - high 1

To switch off LED - low 1

LED connected to Power Rail.

To switch on LED - low 1

To switch off LED - high 1

Bi-colour LEDs often contain both green and red LEDs connected in ‘inverse parallel’.

This means if current flows one way through the device the LED lights green, and if

current flows the other way the LED lights red. Therefore by using the sink/source

capabilities of the PIC Microcontroller it is possible to light the LED in both colours.

To switch on LED in red - high 0

low 1

To switch on LED in green - low 0

high 1

To switch off LED - low 0

low 1

or, high 0

high 1

0V

Pin 1

330R

Pin 1

5V

330R

Pin 0

330R

Red Green

Bi-colour LED

Pin 1




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Output Device 2 - Signal LampOutput Device 2 - Signal LampOutput Device 2 - Signal LampOutput Device 2 - Signal LampOutput Device 2 - Signal Lamp

To interface a signal lamp the standard transistor interfacing circuit is used. Note that if

a different power supply is used for the signal lamp, the 0V rails of each power supply

must be connected to provide a common reference.

If a battery is used as the power supply, it is worth remembering that LEDs draw much

less current than lamps. Therefore, if a simple ‘indicator’ is required, a LED will be a

better solution than a lamp as the batteries will last far longer.

To switch on Lamp - high 1

To switch off Lamp - low 1

0V

Pin 110k

Signallamp

6V

BCX38B

signal lamp

buzzer

Output Device 3 - BuzzerOutput Device 3 - BuzzerOutput Device 3 - BuzzerOutput Device 3 - BuzzerOutput Device 3 - Buzzer

To interface a buzzer the standard transistor interfacing circuit is used. Note that if a

different power supply is used for the buzzer, the 0V rails of each power supply must

be connected to provide a common reference.

If a battery is used as the power supply, it is worth remembering that piezo sounders

draw much less current than buzzers. Buzzers also just have one ‘tone’, whereas a

piezo sounder is able to create sounds of many different tones.

To switch on buzzer -high 1

To switch off buzzer -low 1

0V

Pin10k

Buzzer

BCX38B

6V




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Output Devices 4 - Piezo Sounder & SpeakerOutput Devices 4 - Piezo Sounder & SpeakerOutput Devices 4 - Piezo Sounder & SpeakerOutput Devices 4 - Piezo Sounder & SpeakerOutput Devices 4 - Piezo Sounder & Speaker

A piezo sounder or speaker can be used to produce many different sounds, whereas a

buzzer can only produce a single tone. Buzzers produce a noise when power is

applied, but a piezo or speaker requires a pulsed signal to generate the noise.

Fortunately this is very easy to generate from the microcontroller by using the BASIC

‘sound’ command.

To produce a note of pitch 100, length 50 on pin 1 -

sound 1, (100,50)

To produce a varying noise using variable b1 -

for b1 = 1 to 100

sound 1, (b1,25)

next b1

Pin 1

0V

Pin 1

0V

+

40R

10uF




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Output Devices 5 - Solar & DC “Toy” MotorsOutput Devices 5 - Solar & DC “Toy” MotorsOutput Devices 5 - Solar & DC “Toy” MotorsOutput Devices 5 - Solar & DC “Toy” MotorsOutput Devices 5 - Solar & DC “Toy” Motors

Many projects require the use of a cheap dc motor to create rotational movement.

There are a number of ways motors can be interfaced to the microcontroller.

This circuit uses a darlington transistor to switch the motor on and off. This circuit will

work with ‘solar’ motors, but may not function correctly with cheap dc ‘toy’ motors.

This is because this type of motor introduces a lot of electrical ‘noise’ on to the power

rails. This noise can affect the microcontroller, and in some cases can completely stop

the control program functioning.

0V

Pin 110k

Solarmotor

6V

0V

BCX38B

M1N4001

solar motor

Electrical noise can be reduced by

soldering suppression capacitors

across the motor contacts, as

shown. Use a 220nF polyester

(non polarised) capacitor.

In order to switch medium power motors, a power MOSFET is used instead of a

darlington transistor. The MOSFET circuit is shown below. The device IRF530 is a

suitable power MOSFET to use in this circuit.

0V

Pin

+6V

IRF530

M1N4001




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On many occasions it may be necessary to control two motors. A convenient and

cheap approach would be to use a motor driver IC such as the L293D. This IC will

allow control of two dc motors, using four data lines from the microcontroller.

Naturally, if only one motor is to be controlled then only two output lines are used.

Both inputs low - motor halt

First output high, second output low - motor forward

First output low, second output high - motor reverse

Both inputs high - motor halt

Changing the states of the input pins has the effect of altering the direction of current

flow through the motor, as shown below.

0V 0V

M

Pin 4

L293D

5V

In 1

Out 1

0V

0V

Out 2

In 2

V+

5V

In 3

Out 3

0V

0V

Out 4

In 4

5V

1

8

16

9Pin 5

To V2+

V2+

Motor A M

Pin 6

Pin 7

Motor B

Note that the L293D will become warm with continuous use. A heatsink bonded onto

the top of the chip will help keep it cool.

Current flow




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0V

Pin 010k

BCX38B

1N4001 RL1

M

5VV+

Pin 110k

RL2

1/2

2/11/1

GNDContacts 2/2 not used

relay0V

Pin 110k

BCX38B

1N4001 RL1

M

6V

6Vbattery

One way to prevent electrical noise affecting the microcontroller is to use separate

power supplies for the ‘control’ electronics and the motor. For example, a PP3 battery

may be chosen to power the microcontroller and 4xAA cells to power the motors.

Naturally it will be necessary to ‘link’ the two circuits so that the motor can be

controlled. A relay is an ideal component to do this.

The above circuit will only switch the motor on and off. If the motor is required to run

in both directions (forwards and reverse), two relays can be used as shown.




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Output Device 6 - Unipolar stepper motorOutput Device 6 - Unipolar stepper motorOutput Device 6 - Unipolar stepper motorOutput Device 6 - Unipolar stepper motorOutput Device 6 - Unipolar stepper motor

Stepper motors are very accurate motors that are commonly used in computer disk

drives, printers and clocks. Unlike dc motors, which spin round freely when power is

applied, stepper motors require that their power supply be continuously pulsed in

specific patterns. For each pulse, the stepper motor moves around one ‘step’, often 7.5

degrees (giving 48 steps in a full revolution).

There are two main types of stepper motors - Unipolar and Bipolar. Unipolar motors

usually have four coils which are switched on and off in a particular sequence. Bipolar

motors have two coils in which the current flow is reversed in a similar sequence. Use

of bipolar motors is covered in the next section.

Each of the four coils in a unipolar stepper motor must be switched on and off in a

certain order to make the motor turn. Many microprocessor systems use four output

lines to control the stepper motor, each output line controlling the power to one of the

coils.

As the stepper motor operates at 12V, the standard transistor circuit is required to

switch each coil. As the coils create a back emf when switched off, a suppression diode

on each coil is also required. The table below show the four different steps required to

make the motor turn.

Step Coil 1 Coil 2 Coil 3 Coil 4

1 1 0 1 0

2 1 0 0 1

3 0 1 0 1

4 0 1 1 0

1 1 0 1 0

Look carefully at the table, and notice that a pattern is visible. Coil 2 is always the

opposite (or logical NOT) of coil 1. The same applies for coils 3 and 4. It is therefore

possible to cut down the number of microcontroller pins required to just two by the

use of two additional NOT gates.

stepper motor

+12V




© copyright 2000

Fortunately the darlington driver IC ULN2003 can be used to provide both the NOT

and darlington driver circuits. It also contains the back emf suppression diodes so no

external diodes are required. The complete circuit is shown below.

Before programming, there is another pattern to notice in the stepping sequence. Look

at this table, which just shows coil 1 and coil 3.

Step Coil 1 Coil 3 Change

1 1 1

coil 3

2 1 0

coil 1

3 0 0

coil 3

4 0 1

coil 1

1 1 1

Notice the change from step 1 to step 2, just coil 3 changes. Then look at the next

change - just coil 1 changes. In fact the two coils take it ‘in turns’ to change from high

to low and back again. This high-low-high changing can be described as ‘toggling’

state. This makes the programming very simple by using the BASIC toggle command.

steps: toggle 1 ‘ Toggle pin 1

pause 200 ‘ Wait 200 ms

toggle 2 ‘ Toggle pin 2

pause 200 ‘ Wait 200ms

goto steps ‘ Loop

Note: If stepper motor ‘wobbles’, try adjusting wire polarity.

0V

To 11

Pin 2 ULN

2003

In 1

In 2

In 3

In 4

In 5

In 6

In 7

Gnd

Out 1

Out 2

Out 3

Out 4

Out 5

Out 6

Out 7

Diode

1

8

16

9

To 10NC

Pin 1

1k

1k

+12V

NC

To 1

To 4

1k

1k+5V

BRN

BLK

ORG

YEL

+12V(power supply)

RED

Stepper motor

N.B. colours of steppermotor leads may vary




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Output Device 7 - Bipolar Stepper motorOutput Device 7 - Bipolar Stepper motorOutput Device 7 - Bipolar Stepper motorOutput Device 7 - Bipolar Stepper motorOutput Device 7 - Bipolar Stepper motor

Stepper motors are very accurate motors that are commonly used in computer disk

drives, printers and clocks. Unlike dc motors, which spin round freely when power is

applied, stepper motors require that their power supply be continuously pulsed in

specific patterns. For each pulse, the stepper motor moves around one ‘step’, often 7.5

degrees (giving 48 steps in a full revolution).

There are two main types of stepper motors - Unipolar and Bipolar. Unipolar motors

usually have four coils which are switched on and off in a particular sequence. Bipolar

motors have two coils in which the current flow is reversed in a similar sequence. Use

of unipolar motors is covered in the previous pages.

The bipolar stepper motor has two coils that must be controlled so that the current

flows in different directions through the coils in a certain order. The changing magnetic

fields that these coils create cause the rotor of the motor to move around in steps.

The circuit that is normally used to control one of the coils is shown below. Notice

how there are four ‘control’ transistors, that are switched on in ‘pairs’. Therefore with

two coils there are four control transistor pairs (Q1-Q4) which must be switched on

and off in a certain sequence.

motor coil

Q1

Q2

Q2

Q1

12V

0V

Current flow




© copyright 2000

Notice that as the coils create a back emf when switched off 8 suppression diodes (4

on each coil) are also required.

The table below show the four different steps required to make the motor turn

Step Q1 Q2 Q3 Q4

1 1 0 1 0

2 1 0 0 1

3 0 1 0 1

4 0 1 1 0

1 1 0 1 0

Fortunately the motor driver L293D has been specifically designed to provide this

transistor switching circuit. The L293D contains all 8 transistors and diodes within one

16 pin package.

Four pins from the microcontroller are connected to the four transistor ‘pairs’ via IC

pins 2, 7, 10 and 15.

0V 0V

M

Pin 4

L293D

5V

In 1

Out 1

0V

0V

Out 2

In 2

V+

5V

In 3

Out 3

0V

0V

Out 4

In 4

5V

1

8

16

9Pin 5

To V2+

V2+

Motor A M

Pin 6

Pin 7

Motor B

This sample procedure makes the motor spin 100 steps to the left and then 100 steps

to the right by using two sub-procedures. lstep causes the motor to move one step to

the left, rstep causes the motor to move one step to the right. Variable b1 is used to

store the step position and so should not be used elsewhere in the program.

main: for b3 = 0 to 99 ‘ start a for...next loop

gosub lstep ‘ call left step sub-procedure

next b3 ‘ next loop

for b3 = 0 to 99 ‘ start a for...next loop

gosub rstep ‘ call left step sub-procedure


lstep: let b1 = b1 + 1 ‘ add 1 to variable b1

goto step ‘ goto the lookup table

rstep: let b1 = b1 - 1 ‘ subtract 1 from variable b1

step: let b1 = b1 & 2 ‘ mask lower two bits of b1

lookup b1,(%1010,%1001,%0101,%0110),b2 ‘ lookup code into b2

let pins = b2 ‘ output b2 onto control lines

return




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Output Device 8 - Radio Control ServoOutput Device 8 - Radio Control ServoOutput Device 8 - Radio Control ServoOutput Device 8 - Radio Control ServoOutput Device 8 - Radio Control Servo

Servos are used in most radio controlled

cars and planes to control the steering

mechanism. They are accurate devices that

always rotate the same amount for a given

signal, and so are ideal for use in many

automated machines.

A typical servo has just three connection wires, normally red, black and white (or

yellow). The red wire is the 5V supply, the black wire is the 0V supply, and the white

(or yellow) wire is for the positioning signal.

The positioning signal is a pulse between 0.75 and 2.25 milliseconds (ms) long,

repeated about every 18ms (so there are roughly 50 pulses per second). With a 0.75ms

pulse the servo moves to one end of its range, and with a 2.25ms pulse the servo

moves to the other. Therefore, with a 1.5ms pulse, the servo will move to the central

position. If the pulses are stopped the servo will move freely to any position.

Unfortunately servos require a large current (up to 1A) and also introduce a large

amount of noise on to the power rail. Therefore in most cases the servo should be

powered from a separate power supply, as shown below. Remember that when using

two power supplies the two 0V rails must be joined to provide a common reference

point.

loop: servo 4,75 ‘ move servo to one end

pause 2000 ‘ wait 2 seconds

servo 4,150 ‘ move servo to centre


servo 4,225 ‘ move servo to other end


goto loop ‘ loop back to start

Pin330R

W

R

B

SERVO

6V SUPPLY

V2+

6V 0V




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Output Device 9 - Counter moduleOutput Device 9 - Counter moduleOutput Device 9 - Counter moduleOutput Device 9 - Counter moduleOutput Device 9 - Counter module

The Counter Module is a numeric LCD display module that can be used to show a

‘counter’ value. To increment the counter a pulse (between 1 and 1.5V) must be

applied to the counter pad 3. As the PIC microcontroller operates at 5V a potential

divider formed from resistors must be used to reduce the PIC microcontroller output

signal to 1.5V. As the counter uses it’s own, internal, 1.5V battery, the two 0V rails must

also be connected.

3k3

Pin 1

1k

0V

Counter

1 3

reset

2

0V count

To increment counter: pulsout 1,100

To reset the counter, a second potential divider is added and connected to pin 2.




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Output Device 10 - Seven Segment DisplayOutput Device 10 - Seven Segment DisplayOutput Device 10 - Seven Segment DisplayOutput Device 10 - Seven Segment DisplayOutput Device 10 - Seven Segment Display

Pin 2

Pin 116

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8Pin 0

+5V

0V

Pin 3

fgabcde

B

C

LT

BK

ST

D

A

Gnd

+5V

f

g

a

b

c

d

e

a

g

de

f b

c

4511B

This code example counts through the digits 0 to 9

main: for b1 = 0 to 9 ‘ Set up a for...next loop using variable b1

let pins=b1 ‘ Output b1 onto the four data lines

pause 1000 ‘ Pause 1 second

next b1 ‘ Next

goto main ‘ Loop back to start

A seven segment display contains seven LED

‘bars’ that can be lit up in different

combinations to show the ten digits 0 to 9. In

theory each ‘bar’ could be connected to one

microcontroller output pin, but this would

use up 7 of the 8 available pins!

A better solution is to use a dedicated integrated circuit, such as the CMOS 4511B to

control the seven segment display. This IC controls the seven segment display

according to the binary ‘code’ on the four data lines. This system uses four pins rather

than 7.

IMPORTIMPORTIMPORTIMPORTIMPORTANT NANT NANT NANT NANT NOOOOOTETETETETE - Seven segment displays are available in two types, called ‘commoncathode’ and ‘common anode’. The following circuits will only work with a ‘commoncathode’ type display. Use the manufacturer’s datasheet to determine the pinoutarrangement of the LED bars.




© copyright 2000

Another possible solution is to use the CMOS 4026B to control the seven segment

display. This system uses just two pins to control the display. The reset pin is used to

reset the display to 0, the clock pin is then used to increment the digit up from 0. This

means to display the digit ‘4’ it is necessary to reset and then pulse the clock line 4

times. In reality this means that the display shows the digits 0-1-2-3-4, but, as they are

clocked extremely rapidly, the human eye cannot see the changes, and so the number

‘4’ seems to appear immediately!

This code example uses sub-procedure ‘clock’ to display the digit ‘4’, which is stored in

the variable b1.

‘This is the sub-procedure

clock: pulsout 0,10 ‘ reset display to 0

if b1 = 0 goto endclk ‘ if b1 = 0 then return

for b3 = 1 to b1 ‘ start a for...next loop

pulsout 1,10 ‘ pulse clock line


endclk: return ‘ return from sub-procedure

This is the main code

main: let b1 = 4 ‘ give variable b1 the value 4

gosub clock ‘ call sub-procedure

pause 1000 ‘ wait 1 second

goto main ‘ loop

Pin 016

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

+5V

0V

cbead

Clock

Out

f

g

Gnd

+5V

Reset

c

b

e

a

d

a

g

de

f b

c

4026

fg

Pin 1

To7 segmentdisplay




© copyright 2000

This system can be expanded to two digits by adding a second 4026B IC and a second

seven segment display, as shown in the diagram below. No changes to the code are

required, just give the variable b1 a value between 0 and 99 and the number will be

displayed on the two displays when sub-procedure ‘clock’ is called.

Pin 016

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

+5V

0V

Clock

Out

f

g

Gnd

+5V

Reset

c

b

e

a

d4026B

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

Clock

Out

f

g

Gnd

+5V

Reset

c

b

e

a

d

4026B




© copyright 2000

Output Device 11 - Solenoid & Solenoid ValvesOutput Device 11 - Solenoid & Solenoid ValvesOutput Device 11 - Solenoid & Solenoid ValvesOutput Device 11 - Solenoid & Solenoid ValvesOutput Device 11 - Solenoid & Solenoid Valves

A solenoid consists of a steel plunger inside an electric coil which is wrapped around a

tube. When the coil is energised a magnetic field is created, and this draws the plunger

into the tube. When the coil is de-energised a spring pushes the plunger back out of

the tube.

To control a solenoid the standard MOSFET circuit is used.

The isonic solenoid valve can be used to control air flow through a pneumatic system.

Isonic valves are ideal for battery operated products as operate at a low voltage and

draw much less current than traditional solenoid valves. The standard transistor

switching circuit can be used to drive the isonic valve.

To switch the solenoid on - high 1

To switch the solenoid off - low 1

0V

Pin 110k

1N4001 Solenoidvalve

5V

solenoid

0V

Pin 1

+6V

IRF530

1N4001




© copyright 2000

smartwire

Output Device 12 -Output Device 12 -Output Device 12 -Output Device 12 -Output Device 12 -Smart Wire & Smart SpringsSmart Wire & Smart SpringsSmart Wire & Smart SpringsSmart Wire & Smart SpringsSmart Wire & Smart Springs

Shape Memory Alloy wire or springs are ‘smart’ materials that can be used to create

mechanical actuation (movement). When an electric current is passed through the wire

it heats up and so contracts with a large pulling force. When the current is removed the

wire cools and so expands again (a ‘traditional’ steel spring is sometimes used to pull

the smart wire/spring taut as it cools).

Smart wire or springs draw a relatively large current, and so the standard FET

interfacing circuit should be used to interface to the microcontroller.

To make the wire / spring contract - high 1

To allow the wire / spring to expand again - low 1

0V

Pin 1

+6V

IRF530

1N4001 smartwire




© copyright 2000

INPUT DEVICE INTERFACING

Input Device 1 - SwitchesInput Device 1 - SwitchesInput Device 1 - SwitchesInput Device 1 - SwitchesInput Device 1 - Switches

There are a large variety of switches available, but the majority all have two ‘contacts’

which are either ‘open’ (off) or ‘closed’ (on). The two circuits shown below can be used

with almost all switches.

With this circuit the input pin is low when the switch is open and high when theswitch is closed.

Goto ‘jump’ when switch is open: if pin0 = 0 then jump

Goto ‘jump’ when switch is closed: if pin0 = 1 then jump

5V

0V

10k

1kPin 0

5V

0V

10k

1kPin 0

With this circuit the input pin is high when the switch is open and low when theswitch is closed.

Goto ‘jump’ when switch is open: if pin0 = 1 then jump

Goto ‘jump’ when switch is closed: if pin0 = 0 then jump




© copyright 2000

5V

0V

10k

Pin 0

330R

Pin 1

Switch BounceSwitch BounceSwitch BounceSwitch BounceSwitch Bounce

All mechanical switches ‘bounce’ when the switch opens or closes. This means that the

switch contacts ‘bounce’ against each other before settling. As the PIC microcontroller

operates so quickly it is possible that in some programs the microcontroller may

register 2 or 3 of these ‘bounces’ instead of just registering one ‘push’.

The simplest way to debounce a circuit

is to simply add a time delay (pause

100) after the if... command. If the

section of code after the push is quite

long this time delay will occur

naturally (as the other code

commands are carried out) and so is

unnecessary. However if the code does

not have a long delay, as in the

following example, a pause command

can be used instead.

The following two programs show the effect of switch bouncing. The program should

light the LED on pin1 when the switch connected to pin0 has been pressed more than

5 times. However, the first listing may not work correctly, because the microcontroller

may count ‘bounces’ rather than actual pushes, and so the LED may light prematurely.

init: let b0 = 0

main: if pin 1 = 1 then add

goto main

add: let b0 = b0 + 1

if b0 < 5 then main

high 1

goto main

init: let b0 = 0

main: if pin 1 = 1 then add

goto main

add: pause 100 ‘short delay here

let b0 = b0 + 1

if b0 < 5 then main

high 1

goto main




© copyright 2000

Input Device 2 - PotentiometerInput Device 2 - PotentiometerInput Device 2 - PotentiometerInput Device 2 - PotentiometerInput Device 2 - Potentiometer

A potentiometer (or ‘variable resistor’)

has a spindle that can be moved to change

the resistance value of the potentiometer.

This can be used to measure rotational or

linear movement.

The readADC command is used to measure the value of the resistance by carrying out

an Analogue to Digital Conversion. The value of the resistance is given a ‘value’

between 0 and 255 which is then stored in a variable. After storing the reading in the

variable, the if...then command can be used to perform different functions.

The program below lights three different LEDs (connected to pins 1, 2 and 3),

depending on the analogue sensor reading.

main: readadc 0,b1 ‘ read value on pin0 into variable b1

if b1




© copyright 2000

ANAL

OGUE

VALU

EPIC

ANAL

OGUE

CALIB

RATIO

N BOA

RD

0VIN

5V

0V IN5V

Input Device 3 - Light Dependant Resistor (LDR)Input Device 3 - Light Dependant Resistor (LDR)Input Device 3 - Light Dependant Resistor (LDR)Input Device 3 - Light Dependant Resistor (LDR)Input Device 3 - Light Dependant Resistor (LDR)

A Light Dependant Resistor (LDR) is a resistor that changes in value according to the

light falling on it. A commonly used device, the ORP-12, has a high resistance in the

dark, and a low resistance in the light. Connecting the LDR to the microcontroller is

very straight forward, but some software ‘calibrating’ is required.

It should be remembered that the LDR response is not linear, and so the readings will

not change in exactly the same way as with a potentiometer. In general there is a larger

resistance change at brighter light levels. This can be compensated for in the software

by using a smaller range at darker light levels. Experiment to find the most

appropriate settings for the circuit.

main: readadc 0,b1 ‘ read the value

if b1




© copyright 2000

Input Device 4 - ThermistorInput Device 4 - ThermistorInput Device 4 - ThermistorInput Device 4 - ThermistorInput Device 4 - Thermistor

A thermistor is a resistor that changes in value according to it’s heat. In actual fact all

resistors change in value as they heat up or cool down, but thermistors are

manufactured to show a large resistance change. Connecting the thermistor to the

microcontroller is very straight forward, but some software ‘calibrating’ is required.

It should be remembered that the thermistor response is not linear, and so the

readings will not change in exactly the same way as with a potentiometer. In general

there is a larger resistance change at lower temperatures. This can be compensated for

in the software by using a smaller range at higher temperatures. Experiment to find the

most appropriate settings for the circuit.

main: readadc 0,b1 ‘ read the value

if b1




© copyright 2000

ADVANCED COMPONENT INTERFACING

Advanced Interfacing 1 - LCD DisplayAdvanced Interfacing 1 - LCD DisplayAdvanced Interfacing 1 - LCD DisplayAdvanced Interfacing 1 - LCD DisplayAdvanced Interfacing 1 - LCD Display

A Liquid Crystal Display is an electronic device that can be

used to show numbers or text. There are two main

types of LCD display, numeric displays (used in

watches, calculators etc) and alphanumeric

text displays (often used in devices such as

photocopiers and mobile telephones).

The display is made up of a number of shaped ‘crystals’. In numeric displays these

crystals are shaped into ‘bars’, and in alphanumeric displays the crystals are simply

arranged into patterns of ‘dots’. Each crystal has an individual electrical connection so

that each crystal can be controlled independently. When the crystal is ‘off’ (i.e. when

no current is passed through the crystal) the crystal reflect the same amount of light as

the background material, and so the crystals cannot be seen. However when the crystal

has an electric current passed through it, it changes shape and so absorbs more light.

This makes the crystal appear darker to the human eye - and so the shape of the dot or

bar can be seen against the background.

It is important to realise the difference between a LCD display and an LED display. An

LED display (often used in clock radios) is made up of a number of LEDs which

actually give off light (and so can be seen in the dark). An LCD display only reflects

light, and so cannot be seen in the dark.

LCD CharactersLCD CharactersLCD CharactersLCD CharactersLCD Characters

The table on the next page shows the characters available from a typical LCD display.

The character ‘code’ is obtained by adding the number at the top of the column with

the number at the side of the row.

Note that characters 32 to 127 are always the same for all LCDs, but characters 16 to 31

& 128 to 255 can vary with different LCD manufacturers. Therefore some LCDs will

display different characters from those shown in the table.

Characters 0 to 15 are described as ‘user-defined’ characters and so must be defined

before use, or they will contain ‘randomly shaped’ characters. For details on how to

use these characters see the LCD manufacturers data sheets.




© copyright 2000

0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Row

Val

ue

Column Value

CGRAM(1)

CGRAM(2)

CGRAM(3)

CGRAM(4)

CGRAM(5)

CGRAM(6)

CGRAM(7)

CGRAM(8)

CGRAM(1)

CGRAM(2)

CGRAM(3)

CGRAM(4)

CGRAM(5)

CGRAM(6)

CGRAM(7)

CGRAM(8)




© copyright 2000

Start with a piece of paper, on which one letter is written. Place the card over the paper,

and the letter will be visible because it shows through the ‘display window’. Remove

the card, write another letter, replace the card and they will both be visible. In fact all

of the first sixteen letters will be visible, but the seventeenth will not, as the ‘display

window’ is only wide enough for 16 letters.

Blank ‘paper’

First letter can be seen

a

Next letter can be seen

a b

17th letter cannot be seen as it is ‘outside’ the display window

a b c d e f g h i j k l m n o p q r s t

The operation of the display is quite complex as the display can actually store more

characters than can be displayed at once. A simple model makes this easier to

understand. Imagine a piece of paper with a row of letters written across it. If a piece of

card is taken, which has a ‘window’ cut in it, and the card is placed over the paper, only

some of the letters will be visible. The other letters are still there, it’s just that they

cannot be seen. This is how a LCD display works - it stores a lot of characters, but only

shows a few, through the ‘display window’, at once

20 letters stored in display memory


Only 16 letters can be seen at one time

a b c d e f g h i j k l m n o p

b c d e f g h i j k l m n o p q




© copyright 2000

To be able to see the seventeenth letter it is necessary to move (or ‘scroll’) the display

window one place to the right, but this will also mean that the first letter can no

longer be seen. Advantage can be taken of this ‘moving’ window method to make long

messages appear to scroll across the LCD screen. To do this a long message is written

into the LCD memory, and then the display window is repeatedly scrolled across the

message. This is equivalent to ‘pulling’ the paper under the window to show the long

message. The LCD window does not ‘physically’ move - so to anyone watching the

LCD the letters ‘appear’ to be moving to the right.





On most LCD displays there is memory for 40 characters on each line. Each space in

the RAM memory can be thought of as a ‘box’ which is ready to hold a single

character. Each RAM ‘box’ has a numbered address to describe it. The first line RAM

‘boxes’ are at addresses 128 to 191, the second line RAM ‘boxes’ are from 192 to 255.

16x2 displays have a window that is two lines deep. That means that 16 letters can be

seen on each line. If a character is to be printed on the second line, it is necessary to

move the cursor to the start of line 2. Moving the cursor is very simple; simply send

the RAM address (of the ‘box’ to be moved) as an instruction. Therefore to move the

cursor to the start of the second line, simply send the instruction ‘192’ to the LCD

module. To move the cursor to the fifth position on the second line send the

instruction ‘197’ (=192+5).




© copyright 2000

Note about 16x1 displays...Note about 16x1 displays...Note about 16x1 displays...Note about 16x1 displays...Note about 16x1 displays...Most 16x1 LCDs are in actual fact 8x2 LCDs, but with the ‘second’ line positioned

directly after the first (instead of underneath it). The reason for this is that it is cheaper

to make an 8x2 display than a 16x1 display, as only one ‘LCD controller’ integrated

circuit is required instead of two (look for the black ‘square’ on the rear of the LCD

module). However, this makes 16x1 displays confusing to use, as, after 8 characters

have been printed, the cursor seems to disappear in the middle of the display! If this

type of display is needed, remember that the ‘ninth’ character is actually the first

character of the second line. This problem does not occur with 16x2 or larger displays,

and so, for ease of use, it is probably worthwhile spending the extra money to buy a

16x2 instead of a 16x1.

Connecting The LCD (OPTION 1)Connecting The LCD (OPTION 1)Connecting The LCD (OPTION 1)Connecting The LCD (OPTION 1)Connecting The LCD (OPTION 1)The serial LCD firmware is used to allow serial control of an alphanumeric LCD. This

allows microcontrollers (and microcontroller based systems such as the PICAXE or

Basic Stamp) to visually output user instructions or readings onto a text screen without

the need for a host computer. This is especially useful when working, for example,

with analogue sensors, as the analogue reading can easily be displayed on the LCD

module. All LCD commands are transmitted serially via a single microcontroller pin.

A sample instruction, using the serout command is as follows:

to print the text ‘Hello’ the instruction is simply

serout 7,T2400,(“Hello”)

LCD

serial

LCD

firmware

PICsingle

pin

5V

0V

5V

0V

123456789

181716151413121110

+5V

0V

serialinput

4 MHz4k7

reset

LC

D F

IRM

WA

RE

D7

D6

D5

D4

RS

E

14

13

12

11

4

6

Pin 9

Pin 8

Pin 7

Pin 6

Pin 1

Pin 2

10k0V

Vdd V0 Vss R/W D0 D1 D2 D3

2 3 1 5 7 8 9 10

Pin 5

Pin 18

Pin 17

680R

0V

conn

ectio

ns to

LC

D fi

rmw

are

For more information, see the Serial LCD Firmware datasheet at www.rev-ed.co.uk




© copyright 2000

Connecting The LCD (OPTION 2)Connecting The LCD (OPTION 2)Connecting The LCD (OPTION 2)Connecting The LCD (OPTION 2)Connecting The LCD (OPTION 2)

The LCD has 6 lines that can be connected directly to the PIC microcontroller pins.

However it is a good design practice to add a low value resistor (e.g. 330R) on the

lines to protect against static discharges. The 10k potentiometer connected to pin 3 is

used to adjust the contrast of the display. All unused lines should be tied to ground as

shown.

DB4

DB5

DB6

DB7

SE

RS

14

13

12

11

6

4

6 x 330R

Pin 7

Pin 6

Pin 5

Pin 4

Pin 3

Pin 2

+5V10k

0V

Vdd V0 Vss R/W DB0 DB1 DB2 DB3

2 3 1 5 7 8 9 10




© copyright 2000

A Simple LCD ProgramA Simple LCD ProgramA Simple LCD ProgramA Simple LCD ProgramA Simple LCD Program

The following program will print out the phrase ‘Hello there!’ on two lines of the LCD

display. It uses three sub-procedures called init , wrins and wrchr . These three sub-

procedures carry out all the ‘difficult’ software tasks, and are ‘standard’ sub-procedures

that will not have to be changed. In fact they can be used without understanding how

they work, but it is necessary to know what they do:

init ‘initialises’ the LCD so that it is ready to accept instructions

wrins sends an instruction stored in variable b1 to the LCD module

wrchr sends a character stored in variable b1 to be ‘printed’ on the LCD screen

The three sub-procedures are explained further in the following sections.

EEPROM 0,(“Hellothere!”) ‘ store the text in the EEPROM memory

gosub init ‘ initialise LCD

main: let b1 = 1 ‘ set b1 to ‘clear display’ instruction

gosub wrins ‘ send instruction to LCD

for b3 = 0 to 4 ‘ setup for...next loop (“Hello” - positions 0 to 4)

read b3, b1 ‘ read letter from EEPROM into variable b1

gosub wrchr ‘ send character to LCD


let b1 = 192 ‘ set b1 to ‘start of second line’ position


for b3 = 5 to 11 ‘ setup for...next loop (“there!”-positions 5 to 11)

read b3, b1 ‘ read letter from EEPROM memory into variable b1






© copyright 2000

More Advanced LCD ProgramMore Advanced LCD ProgramMore Advanced LCD ProgramMore Advanced LCD ProgramMore Advanced LCD Program

The following program scrolls the message ‘Hello there everybody!’ across the screen.

As the text is longer than 16 letters, the message is first stored in the LCD memory, and

then the display window is repeatedly scrolled to show all the message.

EEPROM 0,(“Hello there everybody!”) ‘ store the text in the EEPROM memory

gosub init ‘ initialise LCD

start: let b1 = 1 ‘ set b1 to ‘clear display’ instruction


for b3 = 0 to 22 ‘ setup a for...next loop

read b3, b1 ‘ read letter from EEPROM into variable b1



let b1 = 12 ‘ set b1 to ‘hide cursor’ instruction


main: let b1 = 24 ‘ set b1 to ‘scroll display left’ instruction


pause 250 ‘ pause for 0.25s

goto main ‘ loop




© copyright 2000

Standard LCD Sub-ProceduresStandard LCD Sub-ProceduresStandard LCD Sub-ProceduresStandard LCD Sub-ProceduresStandard LCD Sub-Procedures

Before the sub-procedures are studied, it is important to understand how the LCD

module operates. It has two modes of operation, which are called ‘character’ mode and

‘instruction’ mode. The RS pin (pin 2) controls the mode - when high the LCD is in

character mode, when low the LCD is in instruction mode.

The character or instruction is sent as a 4 bit binary number down the data lines (pins

7-4). Every time the Enable pin (pin 3) is ‘pulsed’ the LCD reads the data lines and

prints the character (or carries out the instruction) which is given by the number on

the data lines.

This is not quite the whole story, as each character or instruction is actually made up of

an 8 bit number, which contains a table of all the character and instruction codes. As

there are only four data lines, this 8 bit number is split into two ‘halves’ which are sent

one after the other. The two halves are called the ‘high nibble’ and the ‘low nibble’.

This means that two nibbles are transmitted down the data lines for each character.

1011 0101 = 10110101

high nibble + low nibble = byte

The three ‘standard’ sub-procedures described below perform all of the ‘complicated’

software tasks when using the LCD display. Each sub-procedure is called from the

main program to perform a certain task. These tasks are:

init initialise the display and sets the module to two line operation

wrchr ‘prints’ one ‘character’ onto the LCD screen

wrins writes one ‘command’ to the LCD module.

(This is actually just the wrchr sub-procedure with the addition of one line

that sets the RS line into ‘instruction’ mode at the start of the sub-

procedure).

DB4

DB5

DB6

DB7

SE

RS

14

13

12

11

6

4

6 x 330R

Pin 7

Pin 6

Pin 5

Pin 4

Pin 3

Pin 2

+5V10k

0V

Vdd V0 Vss R/W DB0 DB1 DB2 DB3

2 3 1 5 7 8 9 10




© copyright 2000

init: let pins = 0 ‘ Clear all output lines

let dirs = 252 ‘ Set pins 2-7 as output lines.

pause 200 ‘ Wait 200 ms for LCD to reset.

let pins = 48 ‘ Set to 8-bit operation.

pulsout 3,1 ‘ Send data by pulsing ‘enable’

pause 10 ‘ Wait 10 ms

pulsout 3,1 ‘ Send data again

pulsout 3,1 ‘ Send data again

let pins = 32 ‘ Set to 4-bit operation.

pulsout 3,1 ‘ Send data.

pulsout 3,1 ‘ Send data again.

let pins = 128 ‘ Set to two line operation

pulsout 3,1 ‘ Send data.

let b1 = 14 ‘ Screen on, cursor on instruction

gosub wrins ‘ Write instruction to LCD

return

wrins: low 2 ‘ Change to instruction mode

wrchr: let b2 = b1 & 240 ‘ Mask the high nibble of b1 into b2.

let pins = pins & 7 ‘ Clear the data lines.

let pins = pins | b2 ‘ Put the contents of b2 onto data lines.

pulsout 3,1 ‘ Pulse the enable pin to send data.

let b2 = b1 * 16 ‘ Put low nibble of b1 into b2.

let b2 = b2 & 240 ‘ Mask the high nibble of b2

let pins = pins & 7 ‘ Clear the data lines.

let pins = pins | b2 ‘ Put the contents of b2 onto data lines.

pulsout 3,1 ‘ Pulse enable pin to send data.

high 2 ‘ Back to character mode

return

Note that init uses a let dirs = commands that will affect all 8 pins, not just the 6used by the LCD display. The let pins = commands used by wrins/wrchr will notalter the state of unused pins 0 and 1. Do not use variable b1 or b2 (or w0 or w1) forany other function within a program.

NB. The | character is ‘SHIFT + \’ (next to Z) on a UK keyboard.




© copyright 2000

Using the LCD Instruction setUsing the LCD Instruction setUsing the LCD Instruction setUsing the LCD Instruction setUsing the LCD Instruction set

The codes for the LCD instructions are given below. Each code can be sent to the LCD

module by using the wrins sub-procedure. These instructions can be used to make the

LCD messages more interesting - for instance by flashing the screen or creating

‘moving’ messages which scroll across the screen.

CodeCodeCodeCodeCode InstructionInstructionInstructionInstructionInstruction

1 Clear display and move to the start of the first line

2 Move the cursor and display ‘window’ to the start of the first line

4 Set ‘right to left printing’ mode

5 Set ‘scroll printing to the left’ mode

6 Set ‘left to right printing’ mode

7 Set ‘scroll printing to the right’ mode

10 Turn visual LCD screen off

12 Hide cursor

13 Make cursor flash

14 Turn visual LCD screen (and cursor) on

16 Move cursor left one position

20 Move cursor right one position

24 Scroll display ‘window’ left one position

28 Scroll display ‘window’ right one position

128 Move cursor to the start of the first line

192 Move cursor to the start of the second line




© copyright 2000

Examples:

Clear the display

clear: let b1 = 1 ‘ Set b1 to clear instruction

call wrins ‘ Send it to LCD

Move cursor to the second line

clear: let b1 = 192 ‘ Set b1 to start of second line

call wrins ‘ Send it to LCD

Flash a message 10 times

flash: for b3 = 1 to 10 ‘ Start a for...next loop using

‘ variable b3 Don’t use b1!!

let b1 = 10 ‘ Set b1 to ‘turn visual display

‘ off’ instruction

gosub wrins ‘ Send instruction to LCD

pause 200 ‘ Pause for 0.2 second

let b1 = 14 ‘ Set b1 to ‘turn visual display

‘ back on’ instruction



next b3 ‘ End of for...next loop

Scroll a long message (30 characters long)

scroll: for b3 = 1 to 30 ‘ Start a for...next loop using

‘ variable b3 Don’t use b1!!

let b1 = 28 ‘ Set b1 to ‘scroll display

‘ window right’ instruction



next b3 ‘ End of for...next loop

let b1 = 1 ‘ Set b1 to ‘move scroll window

‘ back to start’ instruction



goto scroll ‘ Loop




© copyright 2000

Advanced Interfacing 2 - Serial Interfacing to a Computer.Advanced Interfacing 2 - Serial Interfacing to a Computer.Advanced Interfacing 2 - Serial Interfacing to a Computer.Advanced Interfacing 2 - Serial Interfacing to a Computer.Advanced Interfacing 2 - Serial Interfacing to a Computer.

Most computers can ‘talk’ to other devices by serial communication. Serial

communication uses a common ‘protocol’ (or code) where characters are converted

into numbers and then transmitted via cables. A computer mouse normally

‘communicates’ serially with a computer, and computer modems work by turning

these numbers into sounds to travel down telephone lines.

As all computers use the same ASCII code for transmitting and receiving characters it is

relatively easy to program the PIC microcontroller to ‘talk’ to any type of computer. All

that is needed is a suitable cable and some very simple electronic circuits.

Connecting to the ComputerConnecting to the ComputerConnecting to the ComputerConnecting to the ComputerConnecting to the Computer

The system we will use requires just three wires between the computer and the

microcontroller. The ground wire provides a common reference, the RX wire sends

signals from the computer to the PIC microcontroller, and the TX wire sends signals

from the PIC microcontroller to the computer.

The best way to make a serial cable is to buy a serial ‘extension’ cable and cut it in

half. This will give two cables with a suitable connector at each end. The diagrams

below show the various wiring connections required.

Computer Communication SoftwareComputer Communication SoftwareComputer Communication SoftwareComputer Communication SoftwareComputer Communication Software

To use this system a communication software package is required for the PC. The

examples below use the Terminal option within the Programming Editor software, but

any communications package can be used.

There are various different protocols that can be used for serial communication, and it

is important that both the computer and the microcontroller use the same setting. The

2400,N,8,1 protocol is used here, which means baud speed 2400, no parity, 8 data bits

and one stop bit. This baud speed is quite slow by modern standards, but is quite

sufficient for the majority of project work tasks. All ‘handshaking’ (hardware or

software) must also be disabled.

PC/RISC PC PC (25 way) Mac

5 3 2 7 3 2 5 4 3

RX = 3TX = 2GND = 5

RX = 3TX = 2GND = 7

RX = 3TX = 5GND = 4




© copyright 2000

PIC Microcontroller Interfacing CircuitPIC Microcontroller Interfacing CircuitPIC Microcontroller Interfacing CircuitPIC Microcontroller Interfacing CircuitPIC Microcontroller Interfacing Circuit

The system described here requires just three wires between the computer and the PIC

Microcontroller. Strictly speaking RS232 serial voltages should be at ±15V, but the

standard 5V from the on-board 5V regulator will be used here. This is not the industry

standard, but works perfectly OK with the majority of computers. This is the circuit

that will be used use for serial communication.

To provide true RS232 voltages another integrated circuit is required. The most

common IC used is the MAX232, which has on-board voltage boosters to create the

required voltage swing. If this setting is used it is necessary to change the N2400

(negative) in all the serial software commands to T2400 (true positive).

0V

10k

22kPin 0

180RPin 1

0V

RX

TXTocomputer To PIC

MA

X 232

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

+

+

+

+

10uF

10uF

10uF

10uF

5V

0V

TX

RX

Pin 0

Pin 1

NC

NC

NC

NC

0V

Computer

PIC

NB. Note polarity - capacitors connected topins 2 and 6 are connected upside down.




© copyright 2000

Transmitting Characters to the Computer ScreenTransmitting Characters to the Computer ScreenTransmitting Characters to the Computer ScreenTransmitting Characters to the Computer ScreenTransmitting Characters to the Computer Screen

The following program will transmit the word ‘Hello’ to the computer screen over and

over again. If the cable is connected and the communication software is operating

correctly, the word will appear every second.

main: serout 1,N2400,(“Hello”) ‘ Send the word ‘Hello’

serout 1,N2400,(10,13) ‘ Send the ‘new line’ instructions.

pause 1000 ‘ Wait one second

goto main ‘ Loop back to the start

Notice that “text” must be enclosed within speech marks. This tells the microcontroller

to convert the text into a string of ASCII codes. Individual ASCII codes can be

transmitted by just giving their numbers. Therefore the two commands below achieve

the same task:

serout 1,N2400,(“Hello”)

serout 1,N2400,(72,101,108,108,111)

Receiving Keyboard Input from the ComputerReceiving Keyboard Input from the ComputerReceiving Keyboard Input from the ComputerReceiving Keyboard Input from the ComputerReceiving Keyboard Input from the ComputerIt can be useful to be able to use a keyboard for people to ‘answer’ questions. This is

achieved by using the serin command as shown below.

main: serout 1,N2400,(10,13) ‘ Start a new line

serout 1,N2400,(“Press a key- “) ‘ Send a message

serin 0,N2400,b1 ‘ Receive a character into variable b1

serout 1,N2400,(b1) ‘ Transmit character back to the screen

if b1=”a” then hot ‘ Is character ‘a’? If yes goto hot

goto main ‘ No, so loop back to start

hot: serout 1,N2400, (10,13,”A is the Hot Key!”)

‘ Send message

goto main ‘ Loop back to start

If this program is run and then a key is pressed on the keyboard, the character will

appear on the screen. This is the microcontroller (not the computer) working. The

keyboard press has been received from the keyboard and then transmitted back to the

screen!




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Characters or numbers?Characters or numbers?Characters or numbers?Characters or numbers?Characters or numbers?

Consider this command: serout 1,N2400,(65)

This will send the ASCII character ‘A’ to the screen.

Now consider this command: serout 1,N2400,(b1)

This will send the character stored in variable b1 to the screen, and so if b1=65, the

character ‘A’ will be sent to the screen.

However, variables are often used to store the answers to mathematical sums, and so it

may be necessary to send the number ‘65’ to the screen rather than the letter ‘A’. To do

this, the microcontroller must be told that a number is to be sent rather than a

character. This is achieved by adding a hash (#): serout 1,N2400,(#b1)

This will send the number ‘65’ (actually the two characters ‘6’ and ‘5’) to the screen

rather than the character ‘A’.

This is a summary of the serial commands used. Remember that the pin number may

have to be changed, and also to the N2400 section to P2400 if the MAX232 interfacing

circuit is used.

serout 1,N2400,(“Hello”)- Sends a message to the screen.

serout 1,N2400,(10) - Sends a direct ASCII instruction to the screen.

serout 1,N2400,(b1) - Sends an ASCII character stored in variable to

the screen.

serout 1,N2400,(#b1) - Sends a number stored in a variable to the

screen.

serin 0,N2400,b1 - Receives an ASCII character from a keypress on

the keyboard and stores it as the ASCII value in

a variable (b1)

serin 0,N2400,#b1 - Receives a real number from the number keys

on the keyboard and stores it in a variable (b1)

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